Polyclonal antibodies against the 40 kDa sterigmatocystin O‐methyltransferase (ST‐OMT) were produced after immunizing rabbits with the purified enzyme. Immunoblot analysis of the crude enzyme preparation fromAspergillus parasiticusstrain 163 revealed one band corresponding to ST‐OMT (∼40 kDa) as well as several other protein bands without enzymatic activity. The antiserum was further purified by passing the ammonium sulfate precipitation‐cut IgG through a column that was armed with proteins isolated from DEAE‐Sephadex gel filtration fraction with immunoreactivity but containing no enzyme activity. Immunoblot analysis revealed that the purified antibodies, after the subtractive affinity chromatography, reacted primarily with one major (∼ 40 kDa) and two minor protein bands of 40–46 kDa size proteins when large amounts of the crude extracts were used. At low protein concentration, only one immunoreactive band of 40 kDa was observed for the crude enzyme preparation. Using the purified antiserum, an indirect ELISA was established for the enzyme detection. Analysis of various fungal extracts showed that the purified antiserum was highly specific for the enzyme. The purified antiserum has also been used successfully for screening the cDNA library in cloning the gene for the enzyme and in monitoring the enzyme produced inEscherichia coliin which this gene(omt‐1)was expressed.